Not Applicable.
Not Applicable
1. Field of the Invention
The invention relates to electronic converter circuits which receive low frequency input power, convert it to DC power at a voltage greater than the peak of the input voltage, and provide output power from a full wave or half bridge output converter. Many electronic lamp ballasts are an example of this kind of device.
2. Description of the Related Art
FIG. 1 is a simplified schematic diagram of a prior art converter having separate stages for each function. A boost converter 11 provides power factor correction on the input current, and has a DC output higher than the peak output from a conventional full wave rectifier 12. An EMI filter 13 blocks high frequency noise from the boost converter from conducting back to the input power line. A down converter 14 matches the boost converter output to the desired level for the input to a full bridge load commutator or inverter 15. A controller 16 controls the switching frequency or times in the boost converter to maintain the input current as sinusoidal as possible, a controller 17 adjusts switching in the down converter responsive to the current drawn by the load commutator or inverter, and a controller 18 sets the commutator 15 frequency and/or switching times to suit the load needs.
FIG. 2 is a schematic diagram of a high power factor converter having a simplified circuit known from U.S. Pat. No. 6,225,755 by the inventor herein. The output converter of this patent does not operate like a typical inverter, in which the output devices switch on and off alternately. Rather, one output power switching device HF3 is switched alternately on and off according to a high frequency pulse width modulated arrangement for a specifiable period of time, and then the other output power switching device HF4 is similarly switched for the specifiable period of time so that the voltage across capacitor C2 at the output is a low frequency square wave. This converter is therefore suitable for driving a high intensity discharge (HID) lamp which preferably should not be driven at a frequency in the range of tens of kilohertz.
U.S. Pat. No. 6,225,755 does not expressly describe the current and voltage switching conditions of the input switches HF1 and HF2, and the output switches HF3 and HF4. Rather, the input switches are described as controlled at a high frequency pulse width modulation arrangement to shape the input inductor current to be in phase with the mains voltage signal, while the output switches are controlled at a high frequency pulse width modulated arrangement to shape the current signal flowing through inductor L2 as a low frequency square wave. The frequency at which the output switches are operated may be a specified frequency. During one output polarity of the square wave, one of the output switches has a high frequency duty cycle chosen to produce a desired average voltage across the other switch, which is deactivated for that half cycle.
One of ordinary skill will understand that, when the input converter is operated under the continuous conduction mode (CCM), the switches are operated such that during a high frequency switching cycle, the inductor current remains continuous, never reaching zero. The current still ramps up linearly and down linearly, but this high frequency component is usually very small compared to the average value of the inductor current.
When the ballast is operated in the discontinuous conduction mode (DCM), the switches are operated such that during a high frequency cycle, the inductor current first ramps up linearly and then down linearly to zero. The current then remains at zero for a period of time before the high frequency switching cycle restarts. This mode of operation is normally used when the switching frequency is fixed to a constant value.
The critical discontinuous conduction mode (CDCM) is the boundary between CCM and DCM. The switches are operated such that the inductor current first ramps up, then ramps down to zero. When the current reaches zero, the switching cycle immediately repeats. Switching depends on the inductor current boundaries, so the switching frequency will depend on the operating conditions of the converter and will vary with these conditions.
An object of the invention is to provide an electronic converter which has improved efficiency because of lossless switching of power semiconducting devices.
Another object of the invention is to provide an electronic converter with a reduced parts count.
A further object of the invention is to provide an electronic lamp ballast having a controllable low to moderate frequency output, which has improved efficiency.
According to the invention, converter efficiency is improved through the use of lossless switching of the power devices in the input stage. The input stage receives low frequency input power and converts this into DC power having a voltage higher than the peak voltage of the low frequency power. Preferably, the converter also has lossless switching of an inverter stage which converts the DC power to output power having a high frequency component.
In this context, lossless switching requires that the voltage across the device""s current terminals must be substantially zero at the time when the device is turned on. Whenever this voltage is not zero, energy is stored in the output capacitance of the switch; if the switch is turned on while that energy is stored, the output capacitance energy is discharged into the switch, and this represents a loss of energy. To produce lossless switching, the main inductor current is used to charge and discharge the switch output capacitances so that the switches can always be turned on at zero voltage. To ensure that switching is lossless, the inductor current may be caused to reverse briefly before switching, thereby removing stored charge in the output capacitance.
An input stage according to the invention provides DC power to negative and positive DC buses. Two input power-switching devices having a switch node between them are connected in series between upper and lower signal lines. A capacitive divider having an intermediate connection is also connected between these signal lines. A boost inductor is connected between the switch node and the intermediate connection. One of the signal lines is connected directly to one of the DC buses, while the other signal line is connected to the other DC bus through a current-sensing resistor.
An output converter stage according to the invention has two buffer capacitors having a switch node between them connected in series between the two DC buses, and also has two converter power switching devices having an output node between them connected in series between these buses. A high frequency inductor through which a load current flows is connected between the switch node and the output node.
Unlike most prior art converters similar to FIG. 1, in each stage the current through the inductor is not sinusoidal, and the circuits are not resonant. Rather, the current through each inductor is triangular and substantially unidirectional for at least a few high frequency cycles. In the input stage the inductor current must go slightly negative; that is, reverse briefly, in order to ensure zero voltage switching while the direction of the triangular pulse is determined by the polarity of the input voltage at that time. This current reversal may be sensed by the current-sensing resistor.
In the inverter stage it is usually not necessary that the current reverse in order to ensure lossless switching. However, if the load is an arc discharge lamp it may be desirable to reverse the current direction periodically.
In a preferred embodiment, the converter is an electronic ballast for an arc discharge lamp. When the output square wave frequency is in the low to mid audio frequencies, this lamp may be an HID lamp. Lamp current is controlled not by changing frequency of an inverter, but by controlling the value of output inductor current at which the inverter switch is turned off. Likewise, the DC voltage is determined by the instant at which the input power switching device is turned off.